Packaging can be produced in various ways and from all kinds of materials. A common way of producing packages involves producing a blank from the packaging material, from which by means of folding and other steps firstly a package sleeve and finally a package are formed. The advantage of this production method, amongst other things, is that the blanks are very flat and can therefore be stacked saving space. In this way, the blanks or package sleeves can be produced at a different place than where the folding and filling takes place. Composite materials are often used, for example a composite consisting of a plurality of thin layers of paper, cardboard, plastic or metal, in particular aluminium. Such packages are widely used particularly in the food industry.
In packaging technology, numerous devices and methods are known, by means of which flatly folded together package sleeves can be unfolded, closed on one side, filled with contents and finally fully closed.
Closing the package sleeves presents a particular challenge because this process must produce a reliable sealing of the package sleeves which must also withstand subsequent transport and other stresses and strains. Closing often takes place in two steps: firstly the package sleeve is heated in the area to be closed (“activated”) and subsequently the opposing sides of the package sleeve are pressed together in the area to be closed (“compressed”). The cohesion between the compressed areas is, for example, achieved by providing an inner plastic layer which becomes viscous during heating and hence brings about adhesion during the subsequent compression. This process is also referred to as “sealing”.
The activation—i.e. heating—of the package sleeves can, for example, be effected by the use of hot air. The advantage of this is that package sleeves consisting of all materials can be heated. In addition, a hot-air blower is very robust. However, the disadvantage is the very high energy requirement, as a result of which the production costs increase considerably.
Alternatively to this, the package sleeves can be activated or heated by means of electromagnetic induction. Inductive heating constitutes a method, in which electrically conductive bodies are heated by eddy current losses which occur in the conductive bodies. For this purpose, an alternating current is conducted through a conductor (the inductor), wherein an alternating magnetic field forms in the surrounding area of the conductor and has an effect on the body to be heated. An eddy current forms in the body to be heated through the alternating magnetic field and the eddy current losses lead to the body being heated.
A change in the magnetic flux density thus leads to the formation of an electrical field which in turn leads to a current flow.
Therefore, a prerequisite for applying this method is for the package sleeves to have electrically conductive areas. Many package sleeves in any case have a layer consisting of metal, in particular consisting of aluminium, since the package contents can be shielded particularly well from light and oxygen by this means. Therefore, such package sleeves can be heated by induction. A layer consisting of an electrically conductive plastic material could likewise be heated by induction. Although the induction initially only causes the electrically conductive layer to be heated, by means of heat conduction and a corresponding arrangement of the layers the inner plastic layer responsible for the adhesion can also be indirectly heated. An advantage of inductive heating is that the heat forms directly in the electrically conductive area and—different from heating by means of hot air—does not have to be transferred by heat conduction. This results in a high level of efficiency, so that the energy requirement and the costs can be kept low. In addition, the amount of heat output can be very precisely controlled.
A device for inductive sealing is known from WO 2014/166765 A1, for example.
A further device for sealing package sleeves is known from WO 00/44619 A1. With this device, the package sleeves to be closed are conveyed in cassettes which are attached to a conveyor belt. The cassettes are designed in such a way that the areas of the package sleeves to be closed jut out from the cassettes on their upper sides and on their undersides. The undersides of the package sleeves are firstly conveyed through a forming station with two opposing rails, by means of which the undersides of the package sleeves are pressed together. Then, the undersides of the package sleeves are conveyed through a sealing device, in which the package sleeves are inductively heated. Following this, the undersides of the package sleeves are firstly conveyed through a compression device and then through a support device. Both the compression device and the support device comprise opposing rollers, between which the undersides of the package sleeves are pressed together.
Inductive heating is effected according to the teaching of WO 00/44619 A1 by conveying the package sleeves with their areas to be heated through a gap in the sealing device. On both sides of the gap, an inductor is arranged having two parallel conductors in each case, so that a mirror-symmetrical structure is produced (cf. FIG. 5 of WO 00/44619 A1). A cooling channel is arranged next to each conductor. A disadvantage of this solution is the large amount of effort required in terms of design for a plurality of inductors with a plurality of conductors and the associated costs. The symmetrical arrangement of the conductors can also—depending on the direction of the current flows—lead to further disadvantages. In the case of opposed flow directions on both sides of the gap the electromagnetic fields of the right or front inductor and of the left or rear inductor superpose one another in such a way that although the magnetic field is strengthened in the area of the gap, the magnetic field lines run predominantly in the vertical direction in the area of the gap and hence parallel to the areas of the package sleeve to be heated. However, for the induction of eddy currents, field lines running perpendicularly to these areas are required. With identical flow directions on both sides of the gap the magnetic field would, by contrast, weaken in the area of the gap.